Silicone compositions have long been used as release coatings, which render a surface or material relatively non-adherent to other materials which would normally adhere thereto. For example, silicone release compositions have found application as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM. and other such substrates are also useful to provide non-stick surfaces for food handling and industrial packaging.
Previously developed silicone release compositions, such as those described in copending U.S. application Ser. No. 267,091, filed May 22, 1981 and now U.S. Pat. No. 4,340,647 and U.S. application Ser. No. 359,480, filed Mar. 18, 1982, now abandoned, have been heat curable, but silicone resins which can be cured with ultraviolet radiation are desirable.
Ultraviolet radiation (UV) is one of the most widely used types of radiation because of its low cost, ease of maintenance, and low potential hazard to industrial users. Typical curing times are much shorter, and heat-sensitive materials can be safely coated and cured under UV radiation where thermal energy might damage the substrate.
Several UV-curable silicone systems are known: U.S. Pat. Nos. 3,816,282 (Viventi); 4,052,059 (Bokerman et al.); and 4,070,526 (Colquhoun) describe compositions wherein .omega.-mercaptoalkyl substituted polysiloxanes react with vinyl-functional silixones in a free-radical process when exposed to UV radiation. These compositions, however, often require scarce or expensive starting materials, have unserviceably slow cure rates, or emit offensive odors (associated with the mercaptan group) which persist in the cured products.
UV-curable silicone resins with epoxy or acrylic functionality have been found recently to have the high degree of reactivity to make them suitable for release applications while avoiding the disadvantages of known UV-curable systems. Epoxy silicone compositions, such as described in U.S. Pat. No. 4,279,717 (Eckberg et al.), are especially advantageous for their rapid curing in the presence of certain onium salt photoinitiators. Acrylic-functional polymers, disclosed in U.S. application Ser. No. 239,297, filed Mar. 2, 1981, now U.S. Pat. No. 4,348,454, can be cured to abhesive coatings under UV radiation in the presence of various free-radical-type photoinitiators.
The above-mentioned U.S. patents and applications are all incorporated herein by reference.